1. Field of the Invention
The invention relates to an X-ray imaging system, including
an X-ray source for emitting an X-ray beam capable of forming an X-ray image after irradiation of an object to be arranged within the X-ray beam,
an X-ray detector for converting the X-ray image into an optical image on an exit screen,
a control unit for controlling the X-ray source, and
a light-optical imaging system for imaging the exit screen on an entrance screen of an image pick-up device, said light-optical imaging system being configured for deflecting a portion of the light emanating from the exit screen toward a photosensitive sensor for forming a control signal to be applied to the control unit. The invention also relates to imaging means suitable for use in such an X-ray imaging system.
2. Description of the Related Art
An X-ray imaging system of the kind set forth is known from European Patent Specification EP 087 843 which corresponds to commonly owned U.S. Pat. No. 4,472,826.
The cited Patent Specification describes an X-ray imaging system in which imaging means are arranged behind the exit screen of an X-ray image intensifier tube and comprise a tandem optical system comprising two lenses, the exit screen being situated in the focal plane of one lens whereas the entrance screen of the image pick-up device, for example comprising a television pick-up tube or a CCD sensor, is situated in the focal plane of the second lens. Between the lenses there is arranged a semitransparent mirror whereby a part of the light beam emanating from the exit screen is projected onto a photographic film of a 100 mm photocamera or onto a 35 mm film of a film camera. Between the lenses of the tandem optical system there is arranged a beam deflection element in the form of a prism whereby a part of the light beam is deflected from the main beam so as to be imaged onto a measurement field selector via a lens. Behind the measurement field selector there is arranged a photosensitive sensor, for example a photodiode, which converts the luminous flux transmitted by the measurement field selective diaphragm into an electric current. The entire exit screen of the X-ray image intensifier tube can be imaged onto the measurement field selector, situated outside the light beam, by mirroring a part of the light beam present between the lenses. The measurement field selector serves to measure the brightness of selected parts of the exit screen of the X-ray image intensifier tube so as to increase or decrease, via a control unit connected to the X-ray source, the voltage or current of the X-ray source when the measured brightness deviates from a desired value. For example, when use is made of a cine film with a film frequency of 50 images per second, the brightness of the exit screen is higher for suitable exposure of the film than in the case of, for example spot film exposures with a film frequency of up to, for example 8 images per second. The image frequencies are dependent on the speed of motion of the objects to be imaged (for example, heart, lungs or vessels). Also in the absence of a photocamera or film camera, a predetermined, constant brightness of the exit screen of the X-ray image intensifier tube is required during irradiation of patients of different thickness in order to ensure adequate illumination of the television pick-up device. When a part of the light beam emanating from the exit screen is deflected and at least a part of this beam is detected, a control signal is formed whereby the voltage and the current of the X-ray source are adjusted via the control unit.
The control signal obtained by mirroring a part of the light beam emanating from the exit screen also serves for exposure timing. Integration of a part of the luminous flux emanating from the exit screen produces a measure for the exposure of the film or the image pick-up device. When the exposure is sufficient, the X-ray source is switched off via the control signal which is formed, for example by an integrated photodiode current.
The known prism for deflecting a part of the light beam emanating from the exit screen has the drawback that it is liable to produce a visible spot in the image. Another drawback consists in that the dimensions and the complexity of the imaging means are increased by the presence of an additional component between the exit screen of the X-ray image intensifier and the television pick-up device.